Britain is playing a key role in developing lethal fungi that could be sprayed over countries illicitly growing cannabis, opium and coca plants, destroying their drug crops, secret US government documents have revealed.

Critics have described the programme as, 'biological warfare' and likened it to the disastrous use of Agent Orange by the US in Vietnam. They warn the fungi will kill food crops, destroy the environment and endanger human life while having little impact on the world drug trade, which will simply move production elsewhere.

US state department documents obtained by The Observer named British scientist Michael Greaves as a key player in its programme to spray a fungus known as Fusarium Oxysporum over Colombia's rainforests in the hope of destroying its coca crops - the source of 80% of the world's cocaine.

Until recently Greaves worked for the government funded Institute of Arable Crop Research in Bristol. He is also the co-ordinator of a secretive UK-funded research project located in the former Soviet republic of Uzbekistan. This is developing another type of fungus aimed at wiping out opium poppies in places such as Afghanistan, where half the world's heroin comes from. The programme is based at the Institute of Genetics in Uzbekistan and run by many scientists who used to work on the Soviet germ warfare programme during the Cold War.

The Foreign Office has provided £100,000 of funding for the project, set up in 1998 under the United Nations Drug Control Programme. In 1999 three scientists from the Uzbekistan Institute came to the Long Ashton research centre in Bristol where Greaves was based. UN documents confirm that since the project began it has been extended to look at fungi to kill cannabis plants. The documents also reveal that, despite widespread concerns from environmentalists and scientists, this fungus has already been field-tested in three Central Asian states: Uzbekistan, Tajikistan and Kyrgistan. Two other states, Kazakstan and Turkmenistan, have so far refused.

Disclosure of the extent of British involvement in what some see as 'biological warfare' in breach in breach of the global Biological Weapons Convention will be highly embarrassing for the government. Only last week Cabinet Minister Mo Mowlam was in Colombia voicing opposition to the use of biological agents there. International criticism of the US plan to use the fungus, dubbed Agent Green, has forced President Bill Clinton to backtrack from tying aid to Colombia with spraying of the fungus.

The US government has not wanted to be seen as the initiator of this programme and persuaded the UN Drug Control Programme to take responsibility. But the US and the UK are the main sources of finance for the multi-million pound programme. US State department documents, signed by the Secretary of state Madeleine Albright, urge the UNDCP to set up testing for 'large scale implementation' of fusarium on coca in Colombia and to get other countries involved 'in order to avoid a perception that this is solely a US government initiative.

The documents say: 'We would urge the formulation of the UN project to take advantage of the extensive US research using fusarium to target illicit coca ... Dr Mike Greaves of the UK ... has already evaluated the host range and exploration data the US has developed.

The fungi work by secreting toxins into a plant's roots, forcing the coca plant or opium poppy to wilt and eventually die. But these fungi can survive in the soil for years and some scientists believe they could be toxic to humans and animals. there is also concern that they spread rapidly, mutate and kill other crops.

In Peru in the late eighties, after an epidemic of fusarium wilt in a coca-growing area caused by this fungus, thousands of peasant families could not grow food. Indeed, a proposal to use the fungus against cannabis crops in Florida was postponed after the state's department of environment protection warned 'it is difficult, if not impossible, to control the spread of fusarium species. The mutated fungi can cause disease in a large number of crops, including tomatoes, peppers, corn and vines'.

Jenny tonge, the Liberal Democrats' international spokesman, said: 'Under no circumstances should the UK support these initiatives. They will damage people, rainforests and pollute the land. Past experience shows that drug production will just move elsewhere. It is time the West looked at how they can curb the demand for drugs from their citizens.

A spokesman for the Foreign Office said: 'There is nothing secret about the project. The UNDCP is seeking safe crop-eradication techniques. this project could lead to a major breakthrough in this field. But these are early days of the project and much careful, lengthy research is still required'. The Observer was unable to contact Greaves who, according to his wife, is in Uzbekistan.